Siamoperadectes is a genus of non-marsupial from the Miocene of Thailand. A member of Peradectidae, it is the first member of its clade known from South Asia, and among the last non-marsupial metatherians.
Description
The type specimen of
Siamoperadectes is a single third upper molar found in the Li Mae Long Basin, northern Thailand.
It displays a rectilinear
Predilambdodonty centrocrista, lacks an hypocone and has a moderately slender lingual part of the molar, all characteristics that most closely connect it to peradectid metatherians. However, it also displays several unique characteristics:
- a deep and narrow protofossa;
- very weak conules;
- an anteroposteriorly compressed protocone;
- a posterior cingulum at the base of the metacone.
The molar is quite small, and in life would probably have belonged to a creature about the size of a modern Monodelphis opossum. Though peradectids have been traditionally considered scansorial, the fact that the relatively closely related Herpetotheriidae were terrestrial
Relationships
Currently,
Siamoperadectes is considered to be a peradectid metatherian, and in particular closely related to
Sinoperadectes and
Junggaroperadectes.
Though described as a
Didelphidae in the original paper,
the current general consensus is that peradectids are outside of crown-group Marsupialia,
and their appearance in the
Late Cretaceous greatly predates the estimated initial divergence within marsupials 45 million years ago.
Ecology
Siamoperadectes is known from the Miocene Li Mae Long deposits, which are rich on a variety of other mammal species such as the
Thaiagymnura equilateralis,
Hylomys engesseri,
Neotetracus butleri and
Scapanulus lampounensis, several
rodents such as
Diatomys liensis, the treeshrew
Tupaia miocenica and several bats, ungulates and carnivorans.
[Mein, P. and Ginsburg, L. 1997. Les mammifères du gisement miocène inférieur de Li Mae Long, Thaïlande : systématique, biostratigraphie et paléoenvironnement. Geodiversitas 19(4):783-844.] So far as known, every other mammal in its environment was a
placental .
Biogeography
Siamoperadectes is the most southerly known peradectid.
The close relations to Chinese peradectids like
Sinoperadectes and
Junggaroperadectes suggest that it had a Laurasian origin rather than having evolved in the Indian subcontinent, and alongside African and Indian
Herpetotheriidae and true marsupials it represents one of several Cenozoic metatherian colonisations of southern landmasses.
Temporal range
Alongside the Chinese
Sinoperadectes,
Siamoperadectes is one of the youngest Laurasian metatherians and certainly one of the last non-marsupial metatherians aside from the South American
Sparassodonta, dating to the mid-Miocene somewhere between 15 and 11 million years ago.
Traditionally, competition with placental mammals has been deemed as a culprit for the ultimate extinction of metatherians outside of
South America and
Australia, but this has been placed into question, especially given in light of the coexistence of both clades through most of the Cretaceous and Cenozoic.
At least herpetotheriids appear to have been reasonably common until the mid-Miocene,
when they suddenly disappear; Asian peradectids followed soon after.
After the extinction of Siamoperadectes, Australian-derived ( Ailurops) colonised Indonesian islands.
